Fender Deluxe 112 Plus Service Manual - Theory of Operation
DELUXE 112 PLUS
THEORY OF OPERATION
Input 1 is a high sensitivity input. Input 2 is a low sensitivity input. Inputs 1 and 2 contain series switches
that connect to +16Vdc via a third series switch located within the Power Amp in jack (J5). From the input
jacks, this series circuit terminates at the gate of JFET Q7. Q7 is part of a muting /turn on delay circuit for
the input of the power amp. Therefore the input signal to the power amp will remain muted until a plug is
inserted into either Input 1,2 or the Power Amp in jack.
The first stage (U2A) sets up the initial response of the amplifier. It provides a second order high pass filter
with a 3dB down point at 60Hz, and a gain of about 3.8. The output of U2A splits to feed the clean and
overdrive channel. For the clean channel, the signal feeds the Treble, Bass and Mid controls that provide
boost and cut respectively. The signal then flows to U7A. U7A is a buffer for the adjustable high pass filter
made up by C59, C60, R108 and R109. The inverted signal at U5A is sensed through R109 and fed back
to the junction of C59 and C60. As the Volume control is turned up, the feedback from R109 provides
–6dB of attenuation at 30Hz. This prevents the speaker from flapping at very low frequencies. U2B is an
Auto-pad circuit. It provides excellent headroom and low noise.
The signal split from U1A feeds the overdrive channel through a special tone shaping circuit located just
prior to U2A. This circuit is a third order bandpass filter with a peak boost at 2.2kHz. It provides the
optimum frequency response for the following distortion circuit. U2A provides the variable gain adjustment
that drives the distortion circuit. It also acts as a bandpass filter with 3dB down points at 90Hz and 4kHz.
U2B is the distortion circuit. LD1 and LD2 provide diode distortion.
The Contour control adjusts the overall tone quality of the overdrive channel by emphasizing or notching
the mid range frequencies. In the fully CCW position the response is fairly flat up to about 2kHz, where a
gentle roll-off occurs. In the fully CW position, the circuit provides a –20db notch at about 1kHz. The
signal then feeds the overdrive volume control and the treble and bass controls.
The signal from the Clean and Drive channels sum at U5A via JFETs Q4 and Q5. The output from U5A
feeds the reverb drive circuit (U6A) and summing amp U5B. U6B is the reverb sensing circuit. The output
of U6B drives the Reverb volume control and then sums with the dry signal at U5B. The output of U5B
drives the preamp out jack, is normalled through the power amp in jack, and feeds the power amp input.
CHANNEL SWITCHING
The AC voltage from the power transformer secondary is tapped off through resistor R82, and presented to
the footswitch jack (Approx 20 V RMS). By rectifying the positive or negative side of the waveform, we
create a DC control voltage that is sensed by comparators U4A and U4B. Diode’s CR20 and CR21 set up
the reference voltages for the comparators. Therefore the control voltage must toggle between +15Vdc
and –15Vdc. The comparator output controls the JFETs and the indicator LEDs. Without the footswitch
connected to the footswitch jack, CR15 rectifies the negative half of the waveform. U4B senses the DC
control voltage, and provides –15Vdc to the gate of Q6, allowing the reverb output to function.
POWER AMP
U1 is a high voltage op-amp that provides voltage gain for the power amplifier. CR3 and CR4 act as
differential protection diodes that prevent the positive and negative inputs of U1 from becoming more than
0.6 volts apart. Bootstrap Capacitors C37 and C38 sense the output through R69. This provides more
voltage swing for the opamp with respect to ground.
Diodes’ CR5-CR8 (BYV26D) make up the Fixed Bias circuit for the output transistors. The Bias Diodes
mount through a hole in the transistor heatsink. This ensures the Diodes will properly track the
temperature of the output transistors. These Diodes were selected because they exhibit a 2mV decrease
in Knee Voltage for every 1 degree (Celsius) increase in temperature. Pin 6 of U1 is at 0 Vdc. Therefore
the Bias Diodes provide 2 voltage drops (+/- 1.2volts) to the Base of Darlington Transistors Q1,Q2, Q10
and Q11.